1. Field of the Invention
The present invention relates to seismic data acquisition, and more particularly to a method and system for transmitting data between multiple remote stations in an array and a data collection station utilizing a linked relay system to communicate therebetween permitting transmission paths to be altered.
2. Description of the Prior Art
Seismic exploration generally utilizes a seismic energy source to generate an acoustic signal that propagates into the earth and is partially reflected by subsurface seismic reflectors (i.e., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). The reflected signals are detected and recorded by seismic units having receivers or geophones located at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Typically, the seismic units or stations are laid out in an array, wherein the array consists of a line of stations each having at least one geophone attached thereto in order to record data from the seismic cross-section below the array. For data over a larger area and for three-dimensional representations of a formation, multiple lines of stations may be set out side-by-side, such that a grid of receivers is formed. Often, the stations and their geophones are remotely located or spread apart. In land seismic surveys for example, hundreds to thousands of geophones may be deployed in a spatially diverse manner, such as a typical grid configuration where each line of stations extends for 5000 meters with stations spaced every 25 meters and the successive station lines are spaced 200 meters apart.
Various seismic data transmission systems are used to connect remote seismic acquisition units to a control station. Generally, the seismic stations are controlled from a central location that transmits control signals to the stations and collects seismic and other data back from the stations. Alternatively, the seismic stations may transmit data back to an intermediate data collection station such as a concentrator, where the data is recorded and stored until retrieved. Whichever the case, the various stations are most commonly hard wired to one another utilizing data telemetry cable. Other systems use wireless methods for control and data transmission so that the individual stations are not connected to each other. Still other systems temporarily store the data at each station until the data is extracted.
In the case of wired stations, typically several geophones are connected in a parallel-series combination on a single twisted pair of wires to form a single receiver group or channel for a station. During the data collection process, the output from each channel is digitized and recorded by the station for subsequent analysis. In turn, stations are usually connected to cables used to communicate with and transport the collected data to recorders located at either a control station or a concentrator station.
In the case of wireless seismic units, each unit communicates with either a central control station or concentrator via radio transmissions. Transmissions are made either directly between each seismic unit and the control station or directly between each seismic unit and the concentrator. To the extent the transmissions are high power, long-range signals, such as between a seismic acquisition unit and a central control station, the transmissions generally require a license from the local governing authority. Units capable of such transmissions also have higher power requirements and thus require larger battery packages. To the extent the seismic acquisition units transmit to a concentrator station utilizing a low power, short-range signal, the transmitting and receiving units must typically have a line of site therebetween.
Illustrative of the prior art is U.S. Pat. No. 6,070,129 which teaches a method and apparatus for transmitting seismic data to a remote collection station. Specifically, an acquisition unit having a geophone attached thereto communicates with a central station either directly by radio channels, or optionally, by means of an intermediate station. To the extent a large number of acquisition units are utilized, the patent teaches that each a plurality of intermediate stations may also be utilized, wherein each intermediate station directly communicates with a portion of the acquisition units. Intermediate stations may function as data concentrators and may also be utilized to control various tasks executed by their respective groups of acquisition units. Whether data is transmitted directly between an acquisition unit and the central station or directly between an acquisition unit and an intermediate station, the transmitting system accumulates seismic data, distributes the data over successive transmission windows and discontinuously transmits the data during successive transmissions in order to lessen variation in seismic data flow.
Similarly, U.S. Pat. No. 6,219,620 teaches a seismic data acquisition system using wireless telemetry, in which a large number of remote seismic acquisition units are grouped together into a plurality of cells and each acquisition unit within a cell communicates directly with a cell access node, i.e., a concentrator, which in turn communicates with a central control unit. This patent teaches that in order to avoid overlap between transmitting seismic units within adjacent cells, adjacent cells utilize different frequencies for communication between units and their respective cell access nodes. In other words, adjacent cells operate at different frequencies so that a particular acquisition unit is only capable of transmitting to the cell access node assigned to its cell.
One drawback to the aforementioned seismic transmission systems of the prior art is that the failure of any one intermediate transmission station or cell access node will prevent communication with a plurality of seismic acquisition units. Furthermore, to the extent an individual unit is prevented from transmitting back to its respective cell access node due to factors external to the unit, the participation and operation of that unit within the array is lost. For example, a unit may lose radio contact with an access point due to a weak signal, weather conditions, topography, interference from other electrical devices operating in the vicinity of the unit, disturbance of the unit's deployment position or the presence of a physical structure in the line of site between the unit and the access point.
Thus, it would be desirable to provide a communication system for a seismic survey array that has flexibility in transmitting signals and data to and from remote seismic units and a control and/or data collection station. The system should be capable of communication between functional seismic units even if one or more intermediate stations fail to operate properly. In addition, the system should be capable of communication between functional seismic units even if a change in environmental or physical conditions inhibits or prevents a direct transmission between a remote unit and its control station.